Wilkinson combiners are used to combine two alternating current (AC) signals with a combined AC signal, whereas Wilkinson splitters are used to split an AC signal into two AC signals. Some Wilkinson combiners may be used as Wilkinson splitters, and vice versa. FIG. 1 shows a lumped traditional Wilkinson combiner 10 according to the prior art. The lumped traditional Wilkinson combiner 10 may include a first port FP and a second port SP for receiving a first AC signal VAC1 and a second AC signal VAC2, respectively, and a common port CP for providing a combined AC signal VCMB, which is based on combining the first AC signal VAC1 and the second AC signal VAC2. At certain frequencies and in certain circuit arrangements, the lumped traditional Wilkinson combiner 10 may provide impedance matching at the first port FP, the second port SP, and the common port CP, and may provide isolation between the first port FP and the second port SP. Such behaviors may be useful in RF circuits, such as quadrature RF circuits, which may split a single signal into two quadrature signals, combine two quadrature signals into a single signal, or both. A traditional Wilkinson combiner 10 may include transmission line elements to achieve the desired impedance matching and isolation behaviors. The lumped traditional Wilkinson combiner 10 may use lumped elements instead of transmission line elements to achieve behaviors that are similar to a transmission line based traditional Wilkinson combiner.
FIG. 2 shows details of the lumped traditional Wilkinson combiner 10 illustrated in FIG. 1. The lumped traditional Wilkinson combiner 10 includes a first inductive element L1 coupled between the first port FP and the common port CP, a second inductive element L2 coupled between the second port SP and the common port CP, an isolation resistive element RISO coupled between the first port FP and the second port SP, a first isolation capacitive element CISO1 coupled between the first port FP and ground, a second isolation capacitive element CISO2 coupled between the second port SP and ground, and an impedance matching capacitive element CM coupled between the common port CP and ground. The first port FP and the second port SP receive the first AC signal VAC1 and the second AC signal VAC2, respectively, and the common port CP provides the combined AC signal VCMB. The first AC signal VAC1 provides a first AC current I1 and the second AC signal VAC2 provides a second AC current I2.
Typically, a magnitude of an inductance of the first inductive element L1 is about equal to a magnitude of an inductance of the second inductive element L2, and a magnitude of a capacitance of the first isolation capacitive element CISO1 is about equal to a magnitude of a capacitance of the second isolation capacitive element CISO2. Therefore, the lumped traditional Wilkinson combiner 10 may be symmetrical and the first port FP and the second port SP may be interchangeable. The isolation resistive element RISO, the first isolation capacitive element CISO1, and the second isolation capacitive element CISO2 form an isolation network. The first inductive element L1 and the second inductive element L2 provide phase shift, and the impedance matching capacitive element CM may help provide impedance matching and isolation.
The lumped traditional Wilkinson combiner 10 may be used in a quadrature RF amplifier circuit to combine quadrature RF signals into a single transmit signal that is fed to an antenna. Such an amplifier circuit may be used in portable wireless devices, such as cell phones, wireless local area network (WLAN) modems, or the like. Such devices may be compact and battery powered; therefore, the lumped traditional Wilkinson combiner 10 may need to be small and efficient. However, the first and the second inductive elements L1, L2 may need to have relatively large inductances for proper operation. The larger inductances require more area and have larger resistive losses. Thus, there is a need for a lumped Wilkinson combiner to have small and efficient inductive elements.